D needs emplacement new

[Namespace via Digitalmars-d]

On Saturday, 25 April 2015 at 18:57:13 UTC, Adam D. Ruppe wrote:
> On Saturday, 25 April 2015 at 18:50:59 UTC, Namespace wrote:
>> Nice name, fits better. But that should be in phobos, don't 
>> you think?
>
> meh, it might be nice to have, but not being in phobos isn't a 
> big deal to me for little utilities like this
I'm sure newcomers (especially those coming from C++) would be 
delighted if they had the possibility to reuse their memory. In 
addition, it could be impossible for them to write such "hack" 
itself.
I already hear the cries: "D wasted memory" and: "I can not reuse 
my memory".
And then it would be less attractive for such people, to switch 
to D.

I would suggest something like:
----
T emplace(T, U...)(ref T obj, auto ref U args) if (is(T == 
class)) {
     if (!obj)
         return null;

     import std.conv : emplace; // temporary, as long it is not in 
std.conv

     enum ClassSizeOf = __traits(classInstanceSize, T);

     void[] buf = (cast(void*) obj)[0 .. ClassSizeOf];
     return emplace!(T)(buf, args);
}

T emplaceOrNew(T, U...)(ref T obj, auto ref U args) if (is(T == 
class)) {
     if (!obj)
         return new T(args);
     return emplace(obj, args);
}
----